As a steel strip continuous annealing device, a large continuous annealing device that anneals a steel strip by multiple passes in a vertical annealing furnace in which a preheating zone, a heating zone, a soaking zone, and a cooling zone are arranged in this order is typically used.
The following conventional method is widely employed in the continuous annealing device in order to reduce water content or oxygen concentration in the furnace, for example upon startup after opening the furnace to the air or in the case where the air enters into the atmosphere in the furnace. The temperature in the furnace is increased to vaporize water in the furnace. Around the same time, non-oxidizing gas such as inert gas is delivered into the furnace as furnace atmosphere replacement gas, and simultaneously the gas in the furnace is discharged, thus replacing the atmosphere in the furnace with the non-oxidizing gas.
However, the conventional method is problematic in that it causes a significant decline in productivity, as lowering the water content or oxygen concentration in the atmosphere in the furnace to a predetermined level suitable for normal operation takes a long time and the device cannot be operated during the time. Note that the atmosphere in the furnace can be evaluated by measuring the dew point of the gas in the furnace. For example, the gas has a low dew point such as less than or equal to −30° C. (e.g. about −60° C.) when it mainly contains non-oxidizing gas, but has a higher dew point such as exceeding −30° C. when it contains more oxygen or water vapor.
In recent years, the demand for high tensile strength steel (high tensile strength material) which contributes to more lightweight structures and the like is increasing in the fields of automobiles, household appliances, building products, etc. The high tensile strength technology has a possibility that a high tensile strength steel strip with good hole expansion formability can be manufactured by adding Si into the steel, and also has a possibility that a steel strip with good ductility where retained austenite (γ) is easily formed can be manufactured by adding Si or Al.
When a high strength cold-rolled steel strip contains an oxidizable element such as Si or Mn, however, the oxidizable element is concentrated on the surface of the steel strip during annealing to form an oxide film of Si or Mn, which leads to problems such as poor appearance and poor chemical convertibility in phosphatization and the like.
Especially in the case of a hot-dip galvanised steel strip, the following problems arise when the steel strip contains an oxidizable element such as Si or Mn: the oxide film formed on the surface of the steel strip impairs the coating property and causes an uncoating defect, or lowers the alloying speed in alloying treatment after galvanisation. Regarding Si, in particular, when an oxide film of SiO2 is formed on the surface of the steel strip, the wettability between the steel strip and the molten metal decreases significantly, and also the SiO2 film constitutes a barrier to mutual diffusion of the steel substrate and the galvanising metal in the alloying treatment, thus impairing the coating property and the alloying property.
This problem may be avoided by a method of controlling the oxygen potential in the annealing atmosphere. As a method of increasing the oxygen potential, for example, WO 2007/043273 A1 (Patent Literature (PTL) 1) describes a method of regulating the dew point from the latter heating zone to the soaking zone to a high dew point greater than or equal to −30° C.